Video rate-enabling probes for atomic force microscopy

ABSTRACT

Method for producing a probe for atomic force microscopy with a silicon nitride cantilever and an integrated single crystal silicon tetrahedral tip with high resonant frequencies and low spring constants intended for high speed AFM imaging.

BACKGROUND OF THE INVENTION

This invention relates to methods for producing probes for use inprobe-based instruments, including applications where high-speed imaging(up to video rate) is desired.

For the sake of convenience, the current description focuses on probesthat may be realized for a particular embodiment of probe-basedinstruments, the atomic force microscope (AFM). Probe-based instrumentsinclude such instruments as AFMs, 3D molecular force probe instruments,high-resolution profilometers (including mechanical stylusprofilometers), surface modification instruments, chemical or biologicalsensing probes, and micro-actuated devices. The probes described hereinmay be realized for such other probe-based instruments.

An AFM is an instrument used to produce images of surface topography(and/or other sample characteristics) based on information obtained fromscanning (e.g., rastering) a sharp tip on the end of a cantileverrelative to the surface of the sample. Topographical and/or otherfeatures of the surface are detected by sensing changes in the probe'smechanical response to surface features and using feedback to return thesystem to a reference state. By scanning the probe relative to thesample, a “map” of the sample topography or other sample characteristicsmay be obtained.

Changes in the probe's mechanical response are typically detected by anoptical lever arrangement whereby a light beam is directed onto thecantilever in the same reference frame as the optical lever. The beamreflected from the cantilever illuminates a position sensitive detector(PSD). As the probe's mechanical response changes, a change in theoutput from the PSD is induced. These changes in the PSD signal aretypically used to trigger a change in the vertical position of the baseof the probe relative to the sample (referred to herein as a change inthe Z position, where Z is generally orthogonal to the XY plane definedby the sample), in order to maintain a constant pre-set value for one ormore of the probe's mechanical responses. It is this feedback that istypically used to generate an AFM image.

AFMs can be operated in a number of different sample characterizationmodes, including contact mode where the tip of the probe is in constantcontact with the sample surface, and AC modes where the tip makes nocontact or only intermittent contact with the surface. These two modesdefine two mechanical responses of the probe that can be used in thefeedback loop which allow the user to set a probe-based operationalparameter for system feedback.

In contact mode the interaction between the probe and the sample surfaceinduces a discernable effect on a probe-based operational parameter,such as the cantilever deflection. In AC mode the effects of interestinclude the cantilever oscillation amplitude, the phase of thecantilever oscillation relative to the signal driving the oscillation,or the frequency of the cantilever oscillation. All of these probe-basedoperational parameters are detectable by a PSD and the resultant PSDsignal is used as a feedback control signal for the Z actuator tomaintain the designated probe-based operational parameter constant.

The feedback control signal also provides a measurement of the samplecharacteristic of interest. For example, when the designated parameterin an AC mode is oscillation amplitude, the feedback signal may be usedto maintain the amplitude of cantilever oscillation constant to measurechanges in the height of the sample surface or other samplecharacteristics.

Some current AFMs can take images up to 100 um², but are typically usedin the 1-10 um² regime. Such images typically require 4-10 minutes toacquire. Many efforts are currently being made to move toward video rateimaging. The reasons for these efforts include the desire to imagemoving samples, to image more ephemeral events and simply to completeimaging on a more timely basis. One important means for moving towardvideo rate imaging is to decrease the mass of the probe, therebyachieving a lower spring constant with a higher resonant frequency.

Currently, conventional probes are 50-450μ in length with springconstants of 0.01-200 N/m and fundamental resonant frequencies, f_(R),of 10-500 kHz. Physical laws put lower limits on the achievableresolution and scan speed of conventional probes, given acceptable noiselevels.

To get the best resolution measurements, one wants the tip of the probeto exert only a low force on the sample. In biology, for example, oneoften deals with samples that are so soft that forces above 10 pN canmodify or damage the sample. This also holds true for high resolutionmeasurements on ‘hard’ samples such as inorganic crystals, since higherforces have the effect of pushing the tip into the sample, increasingthe interaction area and thus lowering the resolution. For a givendeflection of the probe, the force increases with the spring constant,k, of the probe. When operating in air in AC modes where the tip makesonly intermittent contact with the sample surface, spring constantsbelow 30 N/m are desirable. For general operation in fluid, very smallspring constants (less then about 0.1 N/m) are desirable.

To get measurements with higher scan speeds, one wants probes with ahigh f_(R). After passing over a sample feature, the probe response isabout 1/f_(R) seconds for contact mode and Q/f_(R) seconds for AC modes(where Q is the quality factor for the probe). This sets a fundamentallimit on scanning speed: if the response time of the probe is to belowered, the f_(R) must be raised.

The thermal noise of a probe involves fixed noise energy (of order kT)spread over a frequency range up to approximately the f_(R), where k isthe Boltzmann constant and T is the temperature in Kelvin. Thus, thehigher f_(R), the lower the noise per unit band width below f_(R).

The ideal probe for video rate imaging would have a f_(R) in the 5-10MHz range. It would also have a force constant in the 1-40 N/m range.Conventional probes would need to shrink an order of magnitude, toapproximately 5-8 um in length or width, to achieve this goal.

Probes are microfabricated by using semiconductor integrated circuitfabrication techniques as this provides a way to batch produce probeswith consistent cantilever and tip geometries necessary for use withAFMs today. These techniques include, but are not limited to: thin filmdeposition, photolithography with optical masks, Reactive Ion Etching(RIE) with plasma, wet etching of silicon, and wafer-to-wafer bonding.Silicon and silicon nitride are the two primary semiconductor materialsfrom which AFM probes are fabricated. Silicon probes have thickercantilevers which give higher resonant frequencies and force constantsthan silicon nitride probes. This is due to larger thickness variationswhen etching bulk silicon compared to depositing silicon nitride withChemical Vapor Deposition (CVD), forcing silicon processes to stop atthicker cantilevers in order to assure higher yields. One can overcomethese difficulties by using a Silicon-on-Insulator (SOI) wafer, but thisintroduces much higher costs. Silicon nitride probes have duller andshorter tips than silicon probes because silicon nitride is deposited insilicon molds which are difficult to machine and work with.

Current probe fabrication processes limit the ability of the personskilled in the art to reproducibly shrink probe lengths to 5-8 um, aswell as their ability to shrink probe widths to those dimensions whenprobe lengths are also relatively small. This is due to a number offactors, including: (i) photolithography alignment issues whenprocessing both sides of a silicon wafer, (ii) wafer bonding alignmentissues, or (iii) photolithography variations on drastically uneven wafersurfaces. Probe fabrication processes usually incorporate at least oneof these techniques and dimension variations of 5 um are not unusual.Furthermore, shorter probes will require the relatively thin cantileversin order to keep force constants in the range required for AFM. Allthese factors make current processes unviable for the purpose envisionedhere.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improved AFMprobe fabrication process which allows for the production of high-speedcantilevers at or below 8 um in length and width, with integral sharptips and with moderate force constants compatible with AFM at the atomicscale.

The high-frequency low-spring constant probes that are the object of theinvention consist of a handle, a cantilever and a sharp tip. Alternativeembodiments also include probes without tips where one could adddifferent customized tips as needed or use the tipless cantilever as aforce sensor. The handle is formed out of a silicon substrate and has asloping extending edge in a (111) plane which forms an acute angle withthe top of the handle in a (100) plane. The cantilever is formed from asuitable thin film that traverses the sloping (111) edge and extends outin the [110] direction in a (100) plane. The free end of the cantileverhas an integrated silicon tip whose base is attached to the (100) planewith the apex protruding generally out and away from the base.

BRIEF DESCRIPTION OF THE DRAWINGS

All the directions and planes of crystallographic notation used in thefigures use wafer manufacturers' notations and are intended to beequivalent directions and planes.

FIG. 1 is a cross-sectional view showing a silicon (100) wafer withsilicon dioxide and silicon nitride on each side that forms the startingpoint of the first embodiment of the present invention.

FIG. 2 is a cross-sectional view showing the anisotropic etch of thesilicon substrate to form a membrane after patterning an etch mask inthe bottom-side films.

FIGS. 3A and 3B are, respectively, a cross-sectional view andbottom-side plan view of the silicon substrate with membrane after theremoval of the silicon nitride and silicon dioxide layers.

FIG. 4 is a cross-sectional view showing the silicon substrate withmembrane after a deposition of silicon nitride on both sides.

FIGS. 5A and 5B are, respectively, a cross-sectional view andbottom-side plan view showing an etch mask formed on the bottom side ofthe substrate via a shadow mask.

FIGS. 6A and 6B are, respectively, a cross-sectional view andbottom-side plan view showing the patterned silicon nitride on thebottom side of the substrate after the silicon nitride etch and maskremoval.

FIGS. 7A and 7B are, respectively, a cross-sectional view andbottom-side plan view showing the growth of silicon dioxide on theexposed bottom-side of the silicon substrate.

FIGS. 8A and 8B are, respectively, a cross-sectional view andbottom-side plan view showing a cantilever etch mask formed on thebottom side of the substrate via a shadow mask.

FIGS. 9A and 9B are, respectively, a cross-sectional view andbottom-side plan view of the substrate showing the patterned siliconnitride cantilever with a sacrificial silicon dioxide extension.

FIGS. 10A and 10B are, respectively, a cross-sectional view andbottom-side plan view of the substrate showing the anisotropic etch ofthe bottom-side silicon.

FIG. 11 is an enlarged, bottom-side perspective view of the anisotropicsilicon etch with the undercut just past the end of the silicon nitridecantilever.

FIG. 12 is a cross-sectional view after the removal of the sacrificialoxide cantilever extension and subsequent re-growth of silicon dioxideon the exposed silicon surfaces on the bottom-side of the substrate.

FIG. 13A is a cross-sectional view of the substrate after the removal ofthe top-side silicon nitride layer.

FIG. 13B is an enlarged, top-side perspective view of the substrateafter the removal of the sacrificial oxide cantilever extension with anobstructing thin silicon membrane removed for illustrative purposes.

FIGS. 14A and 14B are, respectively, a cross-sectional view and anenlarged cross-sectional view of the substrate after the siliconmembrane is completely removed in an anisotropic etch, leaving only asilicon tetrahedral tip attached to the bottom-side nitride cantileverand bottom-side oxide.

FIG. 15 is an enlarged perspective view of the tetrahedral silicon tipand nitride cantilever after the removal of the bottom-side oxide.

FIG. 16 is a cross-sectional view of the silicon handle, the nitridecantilever, and silicon tip after the removal of the bottom-side oxide.

FIGS. 108A and 108B are, respectively, a cross-sectional view and abottom-side plan view showing a cantilever etch mask formed on thebottom-side of the substrate via a shadow mask, which is part of thesecond embodiment of the present invention.

FIGS. 109A and 109B are, respectively, a cross-sectional view and abottom-side plan view of the substrate showing the patterned siliconnitride cantilever.

FIGS. 110A and 110B are, respectively, a cross-sectional view and abottom-side plan view of the substrate showing the anisotropic etch ofthe bottom-side silicon.

FIG. 111 is an enlarged, bottom-side perspective view of the anisotropicsilicon etch with the undercut etch of the silicon nitride cantilever.

FIGS. 210A and 210B are, respectively, a cross-sectional view andbottom-side plan view of the substrate showing the growth of silicondioxide on the exposed bottom-side of the silicon substrate surroundingthe formed silicon nitride cantilever, which is part of the thirdembodiment of the present invention.

FIG. 211 is a cross-sectional view showing the top-side silicon nitridelayer formed into a tip etch mask which is aligned to the end of thesilicon nitride cantilever on the bottom-side.

FIGS. 212A and 212B are cross-sectional views showing the forming of asilicon tip by etching the exposed top-side silicon surface and stoppingwhen the silicon membrane is completely removed.

FIG. 213 is a cross-sectional view of the silicon handle, the nitridecantilever, and silicon tip after the removal of the bottom-side oxide.

FIG. 308A is a plan view of an optimized cantilever shadow mask with atriangularly shaped handle, which is part of the third embodiment of thepresent invention.

FIG. 308B is a bottom-side plan view of the substrate showing thecritical placement of the optimized cantilever etch mask.

FIG. 311A is a cross sectional view of the substrate showing thedeposition of an etch mask layer on the bottom-side for removal of theexcessive silicon nitride film.

FIGS. 311B and 311C are enlarged, bottom-side perspective views showingthe etching of the cantilever to an optimal shape for use with an AFM.

FIGS. 401, 402 and 403 show another substrate embodiment where thesubstrate is a Silicon-on-Insulator (SOI).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process for producing the high-frequency low-spring constant probesthat are the object of the invention starts with a (100) siliconsubstrate, which has a top-side and a bottom-side. A membrane with a(100) surface bound by (111) planes is formed from this substrate byanisotropically wet etching the bottom-side of the silicon substratewhich has been masked by a film or films formed via any suitablelithography process. The membrane thickness is chosen, using a timedetch, such that a tip of desired height can be formed from the membrane.The bottom-side of the substrate is coated with a film or films suitablefor forming a cantilever. The film or films are formed into thecantilever by aligning a shadow mask to the bottom-side of the siliconsubstrate relative to the rectilinear intersection of a (111) plane andthe (100) plane of the membrane so that a cantilever of controlledlength will extend from this intersection and point in the [110]direction. This length can be further reduced in a process whichundercut etches the cantilever. This exposes the bottom-side siliconmembrane excepting the area covered by the cantilever. The last stepsare to form a tip from the silicon membrane.

One tip-formation process is to etch part of the tip from thebottom-side and part from the top-side of the silicon substrate. In thisprocess the exposed bottom-side silicon is anisotropically etched, aprocessing step which undercut etches the cantilever. The time of theetch will control the height of the tip, while reducing the effectivelength of the cantilever. The undercut etch is stopped after twointersecting {411} planes in the silicon are formed. These planes becometwo facets of the tip. The silicon so exposed is oxidized to protect itduring subsequent etching. The top-side film or films are thenselectively removed to expose the top-side (100) silicon surface. Theexposed silicon is anisotropically etched, completely removing theremaining silicon membrane, except a tetrahedral silicon tip formed onthe free end of the cantilever. This etch exposes the last facet of thetetrahedral tip, a (111) plane. The final step is to selectively removeany remaining oxide on the wafer via wet etching with an appropriateacid.

Another tip-formation process is to etch the tip entirely from thetop-side of the silicon substrate. In this process the exposedbottom-side silicon surrounding the cantilever is oxidized. A top-sideto bottom-side lithography tool can be used to pattern a tip etch maskon the top-side film or films relative to the cantilever on thebottom-side of the silicon membrane. A large number of tip shapes can bemade by appropriately tailoring the tip etch mask shapes and siliconetch processes. For example, one could use plasma (RIE) and/or wetetching processes, both isotropic and/or anisotropic, to create facetedpolyhedron tips or rounded conical tips. In these cases, the etching isstopped when the oxide film and nitride cantilever junction is reachedon the bottom-side of the silicon membrane. The final step is toselectively remove any remaining oxide on the wafer via wet etching withan appropriate acid.

Shadow mask techniques are common to those skilled in the art ofmicromachining. A cantilever shadow mask can easily be produced bymicromachining an aperture with the shape of the desired cantilever in a(100) silicon substrate, or any other suitable substrate. A simplelithography and etch on the top-side of the substrate can make a smalland controllable cantilever-shaped trench. A simple lithography andthru-substrate etch on the bottom-side of the silicon substrate can turnthe trench into the cantilever-shaped aperture.

FIG. 1 shows a cross-sectional view of the first two processing steps ofproducing the high-frequency low-spring constant probes that are theobject of the invention. In the first step, two layers 11 and 12 ofthermal silicon dioxide (referred to herein as oxide) film are grown oneach surface of a monocrystalline (100) silicon substrate 10. Oxide ispreferred because it is pin hole free, uniform, and derives from agenerally clean process. The oxide serves to keep the surfaces of thesilicon substrate 10 clean, and also serves to protect the siliconsurfaces from the Reactive Ion Etch (RIE), which is used to removeportions of silicon nitride film (referred to as nitride herein), asshown in FIG. 2.

In the second processing step of FIG. 1, two layers 13 and 14 of nitridefilm are deposited on the oxide films 11 and 12, respectively. EitherPlasma-Enhanced Chemical Vapor Deposition (PECVD) or Low PressureChemical Vapor Deposition (LPCVD) can be used for this step. The nitridecan be a stoichiometric film (Si₃N₄), though a low-stress variant(Si_(x)N_(y)) is preferred. Similarly, another material can be usedinstead of nitride, such as a polymer or any other semiconductormaterial known to those skilled in the art, as long as it can serve asan etch mask for the wet anisotropic silicon etch shown in FIG. 2.

FIG. 2 shows a cross-sectional view of the next three processing steps.In the first of these steps, conventional lithography followed by a RIEis used to pattern and then etch a rectangular opening through thebottom nitride film 14; the RIE is stopped when it reaches theunderlying oxide film 12. In the second of these steps, a chemicaletchant selective to oxide and non-reactive with silicon or nitride, forinstance HF or Buffered Oxide Etch (BOE), is used to etch therectangular opening through the oxide film 12. In the third of thesethree steps, wet anisotropic silicon etching is used to etch a pit 20into the silicon substrate 10 and form a thin silicon membrane 21 of thedesired thickness by timed etching. Potassium Hydroxide (KOH) is thepreferred etchant, though any other suitable anisotropic silicon etchantwill suffice. The thickness of the membrane 21 will limit the height ofthe silicon tip to be included in the probes resulting from completionof all steps described herein to the thickness of the membrane 21 orsomewhat less. The patterned nitride film 14 resulting from the first ofthese three steps serves as an etch mask for this wet anisotropicetching. The four sidewalls of the pit 20 formed in the siliconsubstrate 10 are {111} crystallographic silicon surfaces.

FIG. 3A shows the removal of the remaining nitride films 13 and 14 andoxide films 11 and 12, leaving the bare etched silicon substrate 10.Concentrated HF (49%) can be used to remove all the remaining filmssimultaneously. Alternatively, boiling phosphoric acid (H₃PO₄) could beused to remove the remaining nitride films 13 and 14, and thereafterdilute HF or BOE can be used to remove the remaining oxide films 11 and12. The remaining silicon substrate 10 can be viewed as three regions ofthe probes that will result from completion of all steps describedherein: 22 will be formed into the silicon handle, 21 will be formedinto the tetrahedral silicon tip, and 10 will be the remaining bulksilicon substrate. FIG. 3B shows a bottom-side plan view of theremaining silicon substrate 10 with the rectangular pit 20 resultingfrom the prior processing steps.

FIG. 4 is a cross-sectional view illustrating the deposition oflow-stress nitride (Si_(x)N_(y)) films 30 and 31 on both sides of thesilicon substrate 10 resulting from the processing step shown in FIG.3A. Either PECVD or LPCVD can be used for this step. The probesresulting from the completion of all steps described herein will includecantilevers made from the nitride film 31. With appropriate changes inprocessing, other materials compatible with silicon processing can beused instead of low-stress nitride, for instance stoichiometric nitride,polymers, metals, composites, or other semiconductor materials known tothose skilled in the art. The thickness of this film is a function ofthe desired specifications of the probes, including the resonantfrequency and spring constant. A critical part of this step is thecareful cleaning of the silicon substrate 10 immediately prior to thenitride deposition. A thin layer of silicon dioxide may be formed duringthe industry-standard diffusion clean process, a wet chemical wafercleaning. This layer can result in the silicon tips of the probesresulting from the completion of all steps described herein etching freefrom the nitride cantilevers in later oxide etch processing steps. Toprevent this result, an oxide etch should be added to the end of thediffusion clean process so that the nitride can be deposited directlyonto the silicon without the presence of a silicon dioxide layer betweenthe two materials.

FIG. 5A shows a cross-sectional view of the process of depositing a masklayer 32 through a micromachined shadow mask (not shown) over a portionof the nitride film 31. The mask layer can be formed from metal,dielectric, polymer, or other materials known to those skilled in theart which will protect a nitride film during a RIE. Use of conventionallithography to define the mask layer would not be appropriate becausethe relatively deep rectangular pit 20 causes severe diffraction for acontact aligner, or focus limitations for projection lithography(stepper). Using e-beam lithography would also be inappropriate for massproduction of these probes due to its extremely high cost. Completion ofthis step defines the length of the probes resulting from the completionof all steps described herein; the length will be the distance frompoint 33 to 34 on the mask layer 32. FIG. 5B shows a bottom-side planview of the surface of the silicon substrate 10 on which the mask layer32 has been deposited over a portion of the nitride film 31.

FIG. 6A shows a cross-sectional view of the RIE patterned nitride film31 where the etch is stopped when the underlying silicon surface isreached. The RIE exposes a portion of the bottom surface of the siliconsubstrate 10, including portions of the membrane 21. FIG. 6B shows abottom-side plan view of the surface of the silicon substrate 10 onwhich the unmasked nitride film 31 has been removed.

FIG. 7A shows a cross-sectional view of the silicon substrate 10 with alayer of oxide 35 grown on the exposed portion of the bottom surface.The nitride films 30 and 31 have prevented growth of oxide under and onthe areas they cover. The nitride-oxide intersection 36 marks what willbe the end of the nitride cantilever and the beginning of thesacrificial oxide cantilever extension. FIG. 7B shows a bottom-side planview of the silicon substrate 10 on which a layer of oxide 35 has beengrown.

FIG. 8A shows a cross-sectional view of the process of depositing acantilever mask layer 40 through a micromachined shadow mask (not shown)over a portion of the nitride film 31 and oxide film 35. Such masklayers have been discussed in connection with the process depicted inFIG. 5A. The mask layer 40 results in the patterning of the probes toresult from completion of all steps described herein. FIG. 8B, abottom-side plan view of the silicon substrate 10, shows the cantilevermask layer 40 pointing in the [110] crystal plane direction. The end 42of the cantilever mask layer 40 extending over the oxide layer 35 isdesigned to be undercut etched during the anisotropic silicon wetetching depicted in FIG. 10A while exposing desired sidewall etchplanes. The cantilever end can be square, but can also be triangularwith two lines along crystallographic directions, for instance the [410]direction or other desired directions, to shorten the undercut etchtime.

FIG. 9A shows a cross-sectional view of the transferred cantileverpattern into the nitride and oxide films, respectively 31 and 35. Thenitride pattern 41 results from removing all the nitride film 31excepting the patterned portion 41 with a RIE. The oxide pattern 45results from removing all the oxide film 35 excepting the patternedportion 45 with dilute HF or BOE solution. The nitride pattern 41 willbecome the cantilever of the probes resulting from completion of allsteps described herein. FIG. 9B, a bottom-side plan view of the siliconsubstrate 10, shows the patterning more clearly.

FIG. 10A shows a cross-sectional view of the silicon substrate 10 aftera wet anisotropic silicon etch. During the etch, two main etch planes{411} slowly undercut etch the silicon underlying the oxide pattern 45.The etch is progressed until the rectilinear intersection 52 of thesetwo undercutting {411} etch planes reaches (or passes depending on theapplication) the junction 36 of the nitride pattern 41 and the oxidepattern 45. The two etch planes will later form two exterior facets ofthe tetrahedral silicon tip of the probes resulting from the completionof all steps described herein. The etch creates an etch pit 50, furtherthinning the unmasked part of the silicon membrane 21, with the bottomof the pit being the thinner silicon membrane 51. FIG. 10B, abottom-side plan view of the silicon substrate 10, shows the result ofthe anisotropic silicon etch more clearly.

FIG. 11 shows an enlarged perspective view from the bottom direction ofthe result of the anisotropic silicon etch. The foreground of FIG. 11shows the nitride pattern 41 and the oxide pattern 45 meeting atjunction 36. The end 43 of the oxide pattern 45 is formed into an arrowpoint where the sides of the arrow 44 align with the [410] siliconcrystal directions. The rectilinear intersection 52 of silicon surfaces53 and 54, which are {411} crystal planes, forms an angle ofapproximately 74° with respect to the [110] silicon crystal direction.The arrow point geometry of the oxide pattern 45 is designed to forcethe {411} crystal planes 53 and 54 to reveal themselves sooner in theundercut etch when compared to the result with a square-end geometry.The edges 44 of the oxide pattern 45 can also be aligned with othercrystal directions, like the [310] or even non-crystal plane directions,in order to optimize the formation of the surfaces 53 and 54 duringundercut etching. However, the final etch profiles of surfaces 53 and 54will still select the {411} crystal plane orientation. In practice,surfaces 55 and 56 are not specific crystal planes. Instead they revealthemselves as multitudinous irregular etch planes. For convenience theyare represented here as single planes.

FIG. 12 shows a cross-sectional view of the silicon substrate 10 afterthe next two steps, an oxide etch step followed by an oxide growth step.In the first step, the oxide pattern 45 is removed with HF or BOEsolution. In the second step, an oxide film 60 is grown on all exposedsilicon surfaces on the bottom-side of the silicon substrate 10. Theresult of this second step is a complete bottom-side etch mask formedfrom the two sections of nitride film, 31 and 41, and the section ofoxide film 60.

FIG. 13A shows a cross-sectional view of the result of removing thetop-side nitride film 30 by a RIE, exposing the top surface of thesilicon substrate 10. FIG. 13B shows an enlarged perspective view fromthe top direction after the step depicted in FIG. 13A. Unlike thedepiction in FIG. 11, here the nitride pattern 41 that will become thecantilever of the probes resulting from completion of all stepsdescribed herein is in the background rather than the foreground. Thesilicon surface in the foreground 21 is shown with the silicon membrane51 stripped away only for illustrative purposes. The growth of the oxidefilm depicted in FIG. 12 extends to the (111) crystal plane and thesurfaces 53 and 55, together with the other (111) crystal plane (notshown) and the surfaces 54 and 56 (also not shown). This film serves asan etch mask in connection with the formation of a tetrahedral silicontip during the wet anisotropic silicon etch depicted in FIG. 14A.

FIG. 14A shows a cross-sectional view of the process of forming asilicon tip 63 on the nitride pattern 41 by etching the exposed top-sidesurface of the silicon substrate 10 with a wet anisotropic silicon etch.The etch is allowed to proceed down to the top of the nitride pattern41, completely removing the thin silicon membranes 21 and 51 except fora small silicon tip 63, exposing the nitride pattern 41 and the oxidefilm 60. The handle 22 of the probes resulting from completion of allsteps described herein is now separated from the remainder of thesilicon substrate 10. FIG. 14B shows a zoomed-in cross-sectional view ofthe step depicted in FIG. 14A. The handle 22 of the probe is not shown.Surface 65 of the tip of the probe is formed during the wet silicon etchand is a (111) crystal plane. The oxide layer 60 will be removed withdiluted HF or BOE solution to release the tip 63 and the nitride pattern41, which is now the cantilever. The bond between the tip 63 and thecantilever 41 will hold during the etch of oxide film 60 because thereis no intermediate oxide layer between the nitride cantilever 41 and thesilicon tip 63 as discussed in connection with the process depicted inFIG. 4.

FIG. 15 shows an enlarged perspective view of the silicon nitridecantilever 41 with a tetrahedral three-sided silicon tip 63. The surface65 is a (111) crystal plane, the slowest etch plane in wet anisotropicsilicon etch. The surfaces 53 and 54 are {411} crystal planes whoserectilinear intersection 52 forms an angle of approximately 74° with thenitride cantilever 41.

FIG. 16 shows a cross-sectional view of the final probe. The siliconhandle 22 of the probe, the nitride cantilever 41 and the single crystalsilicon tetrahedral three-sided tip 63 are shown.

FIGS. 108A through 111 show cross-sectional, bottom-side plan andenlarged perspective views of another embodiment for producing thehigh-frequency low-spring constant probes that are the object of theinvention. In this embodiment, the processing steps depicted in FIGS. 5Athrough 11 of the first embodiment, which are in part necessary forformation of the sacrificial oxide extension (45 of FIGS. 9A through 11)are omitted and the processing steps depicted in FIGS. 108A through 111substituted in their stead. The entire probe fabrication process forthis embodiment follows the processing steps of the first embodimentexcept for the omissions and substitutions just referred to.

FIGS. 210A through 213 show cross-sectional and bottom-side plan viewsof another embodiment for producing the high-frequency low-springconstant probes that are the object of the invention. This embodimentbegins with the processing steps depicted in FIGS. 1 through 4 of thefirst embodiment, followed by the processing steps depicted in FIGS.108A through 109B of the second embodiment and finished with theprocessing steps depicted in FIGS. 210A through 213. FIGS. 210A and 210Brepresent the growth of a silicon dioxide film on the exposedbottom-side silicon resulting from completion of the processing stepdepicted in FIGS. 109A and 109B. The next step, shown in FIG. 211, is alithography step on the top-side of the substrate which is aligned withthe end of the nitride cantilever 43 on the bottom-side of thesubstrate. This may be accomplished with a lithography tool known tothose skilled in the art. Using the tool and RIE, the nitride layer 30is formed into a tip mask 81 which is selectively stopped on the siliconmembrane 21. The tip mask can be any number of shapes, including circlesand polygons with any number of sides. FIGS. 212A and 212B show theetching of the exposed silicon on the top-side of the substrate. Theetching can be done with a wet isotropic chemistry, a wet anisotropicchemistry or a plasma RIE. The idea is that any number of different tipsshapes which may be useful for different AFM imaging needs can beproduced with a tip etch process that is done entirely from the top-sideof the silicon substrate. FIG. 213 shows a cross-sectional view of thefinal probe after any remaining oxide 80 is selectively removed with awet HF based etchant. The silicon handle 22 of the probe, the nitridecantilever 41 and the single crystal silicon tip 82 are shown.

FIGS. 308A through 311B show cross-sectional, bottom-side plan andenlarged perspective views of another embodiment for producing thehigh-frequency low-spring constant probes that are the object of theinvention. This embodiment is a cantilever optimization technique thatmay offer benefits for use with AFMs. AFMs often include integratedoptical microscopes so that a probe tip can be landed on a specific spoton a sample of interest. This embodiment results in a cantilever withthe tip end having a triangular point conforming with the outermostfacets of the tip, thereby giving an optimal plan view of the probewhich allows an AFM operator to land the tip on a specific spot of thesample.

FIG. 308A shows a plan view of a shadow mask 341 which will form acantilever etch mask on the probe substrate. The cantilever shapedaperture 340 in the shadow mask 341 is wide at the base, narrows to arectangle along its length and terminates in a triangular end. Thisembodiment begins with the processing steps depicted in FIGS. 1 through4 of the first embodiment, followed by the processing steps depicted inFIGS. 108A through 110B of the second embodiment, except that here theshadow mask 341 results in the shape 340 depicted in FIG. 308A insteadof the shape depicted in FIGS. 108B, 109B and 110B. FIG. 308B shows abottom-view of the substrate equivalent to that depicted in FIG. 108Bwhich is the result of completing the processing steps referred to inthe preceding sentence. FIG. 308B shows a critical alignment 47 with thewide part of the mask landing on the flat underside of the (100)membrane just past the (111) plane of the probe substrate. Aftercompletion of the processing step depicted in FIGS. 110A and 110B, thisembodiment continues with the processing step shown in FIG. 311A. Inthis step, chrome/gold or any other suitable thin film material, isdeposited 90 on the bottom-side of the substrate to be used as a shapingetch mask for the cantilever. The substrate can then be exposed to a wetetchant, like hot Phosphoric Acid, or a RIE selective to the cantilevermaterial in order to remove the extraneous overhanging cantilevermaterial. The cantilever material will be removed from any place wherethere is exposed cantilever film. The chrome/gold layer can then beselectively removed using appropriate chemical etchants. FIGS. 311B and311C are enlarged bottom-side perspective views of the cantilevermaterial that will be removed during the etch, the former being beforethe etch and the latter after the etch. The wide portion of thecantilever base 48 that connects to the silicon handle 22 was necessaryto prevent the silicon handle from forming erratic undercut etch planesnear the cantilever base during the tip etch steps depicted in FIG. 110.If a wide cantilever base was not used, the undercut etch planes wouldcreate notches in the cantilever where it extends from the (111) planeof the silicon handle 22, the reason being that arbitrary silicon planeswill be exposed when nitride film is removed by RIE during the processstep depicted in FIG. 109 due to poor selectivity of nitride to siliconduring a RIE.

After completion of the processing steps just outlined, this embodimentis finished with the processing steps depicted in FIGS. 12 through 16.

FIGS. 401 through 403 show cross-sectional views of another embodimentfor producing the high-frequency low-spring constant probes that are theobject of the invention where the starting substrate is aSilicon-on-Insulator (SOI) substrate. This substrate is suitable for allprocessing steps depicted in any of the previous figures except that aSOI wafer is substituted for the silicon substrate. FIG. 401 shows thestarting SOI substrate 110 after oxide films 11 and 12 and nitride films13 and 14 have been added. Note that the silicon membrane 21 is part ofthe starting SOI substrate and is separated from the bulk silicon 10 bythe oxide insulator 15. FIG. 402 shows the patterning of the bottom-sidenitride film 14 and oxide film 12 followed by the wet anisotropicsilicon etch. In this case, the wet etch automatically stops when theburied oxide layer 15 is reached. It is the extra oxide layer 15 that isthe key advantage to the SOI substrate as it will keep the siliconmembrane 21 free of etch defects when compared to the results given bythe counterpart processing step depicted in FIG. 2. Etch defects, whichare commonplace when wet etching silicon, can propagate thru to thefinal cantilever surface in standard silicon substrate processing, andadversely affect the probe's performance. FIG. 403 is the SOIcounterpart to FIG. 3 where the nitride films 13 and 14 and oxide films11 and 12 have been removed from the substrate with appropriate acids.Note that the buried oxide insulator layer 15 is not compromised duringthe oxide etch due to its minimal exposed surface area.

The described embodiments of the present invention are only consideredto be preferred and illustrative of the inventive concept. The scope ofthe invention is not to be restricted to such embodiments. Various andnumerous other arrangements may be devised by one skilled in the artwithout departing from the spirit and scope of the invention.

1. A method of making a probe, comprising: A) providing a crystallinesubstrate of a first material having a first side and a second side thatis opposite to the first side; B) forming a first layer over the firstside of the substrate; C) undercutting a first portion on the first sideof the substrate beneath at least a portion of the first layer, to forma step in the first portion of the substrate and a space between aportion of the first layer and the first portion of the substrate; andD) removing a second portion of the second side of the substrate oversaid step to leave a tip of the first material connected to the firstlayer, wherein at least a portion of the first layer forms a cantileverattached to a remaining portion of the substrate.
 2. The method of claim1, wherein the substrate is a silicon substrate and the first materialis silicon.
 3. The method of claim 1, wherein the substrate is asilicon-on-insulator substrate and the first material is silicon.
 4. Themethod of claim 1, wherein the first side of the substrate has an unevensurface.
 5. The method of claim 4, further comprising etching a pit inthe first side of the substrate prior to forming the first layer to forma membrane in the substrate defined by the etched pit.
 6. The method ofclaim 5, wherein a height of the tip is equal to or less than athickness of the membrane.
 7. The method of claim 1, further comprisingpatterning the first layer prior to the step of undercutting.
 8. Themethod of claim 7, wherein the step of patterning comprises depositingat least one mask layer over the first layer and removing an unmaskedportion of the first layer.
 9. The method of claim 8, wherein the stepof depositing comprises depositing at least one mask layer through ashadow mask aligned along the first side of the substrate.
 10. Themethod of claim 8, further comprising etching a pit on the first side ofthe substrate to form a membrane in the first portion of the substratedefined by the etched pit and a non-membrane region and wherein the stepof depositing comprises depositing at least one mask layer on themembrane and the non-membrane region such that a width of the mask isgreater on the non-membrane region than on the membrane.
 11. The methodof claim 10, wherein the mask has a triangular shape on the non-membraneregion.
 12. The method of claim 7, wherein said step of patterningdefines a length of the probe.
 13. The method of claim 1, wherein thefirst layer comprises a cantilever layer.
 14. The method of claim 13,wherein the cantilever layer comprises a silicon nitride, a polymer, ametal or a composite cantilever.
 15. The method of claim 14, wherein thecantilever layer comprises a low stress silicon nitride cantilever. 16.The method of claim 13, wherein the first layer further comprises acantilever extension layer.
 17. The method of claim 16, wherein thecantilever layer is a silicon nitride layer and the cantilever extensionlayer is a silicon oxide layer.
 18. The method of claim 16, wherein thestep is formed at a border between the cantilever layer and thecantilever extension layer.
 19. The method of claim 18, furthercomprising removing the cantilever extension layer after the step ofundercutting.
 20. The method of claim 1, wherein the substrate is asilicon substrate, the first material is silicon, the step comprises twointersecting {411} Si crystal plane surfaces, and the formed tipcomprises the two intersecting {411} Si plane surfaces intersecting witha {111} Si plane surface.
 21. The method of claim 20, wherein removing asecond portion of the second side of the substrate comprises forming the{111} Si plane of the tip.
 22. The method of claim 1, whereinundercutting a first portion on the first side of the substratecomprises anisotropic etching.
 23. A method of making a probe,comprising: A) providing a crystalline substrate of a first materialhaving a first side and a second side that is opposite to the firstside; B) forming a first layer over the first side of the substrate; C)etching a pit in the first side of the substrate prior to forming thefirst layer to form a membrane in the substrate defined by the etchedpit; and D) removing a portion of the substrate from the second sidethereof to expose the first layer and to form a tip of the firstmaterial connected to the first layer, wherein at least a portion of thefirst layer forms a cantilever attached to a remaining portion of thesubstrate; wherein the first side of the substrate has an unevensurface.
 24. The method of claim 23, wherein the substrate is a siliconsubstrate and the first material is silicon.
 25. The method of claim 23,wherein the substrate is a silicon-on-insulator substrate and the firstmaterial is silicon.
 26. The method of claim 23, wherein a height of thetip is equal to or less than a thickness of the membrane.
 27. The methodof claim 23, further comprising patterning the first layer prior to thestep of removing.
 28. The method of claim 23, wherein the step ofremoving includes depositing a tip mask over the second side of thesubstrate such that the tip mask is aligned with an end of the firstlayer located over the first side of the substrate.
 29. The method ofclaim 28, wherein the tip mask has a circular or a polygonal shape. 30.The method of claim 28, wherein the tip mask is silicon nitride.
 31. Themethod of claim 23, wherein said step of removing comprises wetisotropic etching, wet anisotropic etching or plasma reactive ionetching.
 32. The method of claim 23, wherein the first layer is siliconnitride.
 33. The method of claim 23, wherein the tip is a polyhedron tipor a rounded conical tip.
 34. A method of making a probe, comprising: A)providing a crystalline substrate of a first material having a firstside and a second side that is opposite to the first side; B) forming afirst layer over the first side of the substrate; C) patterning thefirst layer prior to a step of removing a portion of the substrate; andD) removing a portion of the substrate from the second side thereof toexpose the first layer and to form a tip of the first material connectedto the first layer, wherein at least a portion of the first layer formsa cantilever attached to a remaining portion of the substrate; whereinthe step of patterning comprises depositing at least one cantilever masklayer over the first layer and removing an unmasked portion of the firstlayer to form the cantilever.
 35. The method of claim 34, wherein thestep of depositing comprises depositing at least one cantilever masklayer through a shadow mask aligned along the first side of thesubstrate.